Eventually the question becomes "if these guest fighters are successful, at what point do they consider going north through the contested border and attacking Russian (or, more likely, Belurussian) cities?"
Ukraine should even refrain from retaking Crimea militarily if things go well, unless it's virtually bloodless. This war has to end ASAP, and it won't end ASAP if Russians suddenly have a non-absurd reason to defend their land.
This is an important point. This is why we should keep reiterating to the Russian people that the sanctions are not to spite them and will be revoked immediately if Putin is gone.
Kicking a poorly motivated invasion force out of the Ukraine is one thing.
Wreaking havoc in Russia? Quite another.
Now, Belarus, on the other hand, I could see toppling over in such a way (unless propped up by Russia)
Not sure what to make of it, but people are definitely thinking about this.
Could be all over the ideological spectrum. Surely quite a few Neo-Nazis, yes, but: If you're a fervent anti-Nazi who sees Putin as rather equivalent to a fascist (I pretty much do; a kind of Duginian pan-Slavo-fascism), you could AFAICS go fight against the Russians on those grounds too.
UA forces are doing a good job holding out and fighting defensively, but a genuine attack will require serious concentrations of troops and equipment. Outside of local counterattacks, or maybe special forces raids (blow up rail lines, etc), large scale assaults probably aren't feasible.
You only need a few NLAWs and a yoga mat to sleep on if you're fighting in Kherzon, but you'll need a lot of fighting vehicles and gas if you want to launch an offensive. See also: Russian logistical problems.
It's like a mouse attacking a cat. The mouse might get lucky and survive an attack by a cat, but it has no chance of killing the cat.
Ukraine is also likely to lose all Western support if it attacked Russia.
If they survive at all (which is still very doubtful), they'll likely lick their wounds and try to rebuild their shattered country rather than go on the offensive in Russia proper.
Power tends to corrupt, and absolute power corrupts absolutely.